Frequency vs dB

Frequency 28-24k @ -6dB
Freq. 35Hz-20kHz +/- 3 dB
Frequency Response (–3dB) 32Hz – 40kHz
These are from specs of diff l.speaker brands.

Could any one explain how will this affect in perfomance?

Regards
Ajith

First let us quickly understand what a how a speaker functions.

Speakers consist of one or more driver units in a box. The driver is constructed of a metal frame to which is attached a cone, made of paper or plastic and occasionally metal. At the rear end of the cone is attached a coil of wire (the "voice coil") wound around an extension of the cone, called a "former". The two ends of the voice coil are connected to the crossover network, and the crossover network is connected to the speaker binding posts on the rear of the speaker enclosure. The voice coil is suspended inside a permanent magnet so that it lies in a narrow gap between the magnet pole pieces and the front plate. The voice coil is kept centered by a "spider" that is attached to the frame and to the voice coil. A rear vent allows air to get into the back of the driver when the cone is moving, but a dust cap on the cone keeps air from getting in through the front. A rubber surround at the outer edge of the cone allows for flexible movement. In the case of a tweeter, the cone is very light, perhaps made of silk, and is glued directly to the voice coil. It isn't attached to a frame or rubber surround because it needs to be very low mass in order to respond quickly to high frequencies.

When the musical electrical signal from the amplifier passes through the voice coil, the voice coil turns into an electromagnet. Depending on which way the current is travelling in the voice coil, the north and south pole of the magnetic field will be at one end of the voice coil or the other. The permanent magnet has a north and south pole as well. Its magnetic field will push the coil (and the attached speaker cone) outward if the north and south poles of the two magnetic fields are lined up together (north to north, and south to south), or pull the voice coil inward if they are lined up oppositely (north to south, and south to north). So, as the electrical signal from the amplifier, which is a representation of the original musical waveform, passes through the voice coil, and changes direction with the musical waveform, the voice coil and attached speaker cone are driven outward and pulled inward in time with the music. The speaker cone pushes or pulls air in the room, which translates to increases or decreases in air pressure at your eardrums, and there you have it: music.

Speakers have created air pressure. Human ears do just the reverse. Human ears have diaphragm that are like microphone. When you put pressure on the diaphragm, it vibrates and creates electrical signals that are sent to neurons in our brains. These neurons convert the signals to what we perceive as sound.

The human ear hears sound as it is sensitive to pressure in the air. It does not have a flat spectral response. In other words, it's efficiency of detection or reaction to sound pressure), is a function of the frequency or wavelength of the sound signal. Sound pressure is often frequency weighed such that the measured level will match the perceived level. When weighed this way, the measurement is referred to as a sound level.

When you say perceive, you are referring to a comparison or a ratio. To do this you must have a reference level. The commonly used reference sound pressure in air is 20 micropascals (20 µPa) (rms), which is usually considered the threshold of human hearing (roughly the sound of a mosquito flying 3 m away). The issue is this is a very small unit (as it is 2 ten billionths of an atmosphere) and difficult to represent. Thus to measure sound pressure levels, a logarithmic ratio is used.

Thus sound pressure level (SPL) or sound level is a logarithmic measure of the rms sound pressure of a sound relative to a reference value. It is measured in decibel (dB).

The formula for calculating differences in sound pressure is

Lp=10Log10 (P~2 rms / P~2ref) = 20log10 (Prms/Pref) = dB

Where where p(ref) is the reference sound pressure and p(rms) is the rms sound pressure being measured. [Please see attachment for a representation of the formula is correct mathematical form.]

Most measurements of audio equipment will be made relative to this level, meaning 1 pascal will equal 94 dB of sound pressure.

For instance, suppose we have two loudspeakers, the first playing a sound with power P1, and another playing a louder version of the same sound with power P2, but everything else (how far away, frequency, etc) kept the same.

The difference in power between the two is defined to be

10 log (P2/P1) dB where the log is to base 10.

If the second produces twice as much power than the first, the difference in dB is 10 log (2/1) = 10 log 2 = 3 dB.

If the second had 10 times the power of the first, the difference in dB would be 10 log (10/1)= 10 log 10 = 10 dB.

Remember that decibels measure a ratio. 0 dB occurs when you take the log of a ratio of 2. So 0 dB does not mean no sound, it means a sound level where the sound pressure is equal to that of the reference level. This is a small pressure, but not zero. It is also possible to have negative sound levels: - 20 dB would mean a sound with pressure 10 times smaller than the reference pressure, ie 2 µPa.

Let us see how a speaker specification is shown. If you look at the B&W 683, there are two rows of information.

Frequency response 38Hz - 22kHz ±3dB on reference axis (EFFICIENCY)
Sensitivity 90dB - (2.83V, 1m) (SENSITIVITY)

Both use the same unit of measurement - dB. Well let us not get confused. Decibel or dB is a unit of measurement that is used in both sound signal and electrical signals. Unfortunately since the unit of measurement is the same, a loudspeaker's sensitivity appears to be universally confused with its efficiency.

Sensitivity is strictly defined as how much acoustic power the loudspeaker puts out for how much electrical power it is being driven with. For example, if you feed a loudspeaker with 10 electrical watts, how many acoustic watts of sound does it produce? The answer is "not many," a typical moving-coil loudspeaker being about 1% 'efficient'.

Efficiency, on the other hand, is expressed as sound-pressure level produced by a speaker at a specific distance, 1m, for 1W input; ie, in decibels watt meter (dB/W/m) or simply dB. This is not that simple as it sounds as a loudspeaker's efficiency is dependent upon both impedance and frequency.

To simplify matters, the impedance of a loudspeaker is always assumed to be 8 ohms at 1000 Hz, at 2.83V, As per ohms law, (Power = V x V/R = (2.83 x 2.83)/8 = 1W.), you are now feeding 1 watt to the loudspeaker.

So there we have it. The base frequency for measuring the efficiency of a loudspeaker is 1000Hz or 1KHz.

The efficiency of loudspeakers are measured in what is called a anechoic chamber. This is a room where sound-absorbing materials on every surface soak up every sound emitted by the speaker. The room is therefore removed from the picture and the only sound that reaches the measuring microphone is therefore that from the speaker. A SPL meter is kept 1 meter from the speaker. The loudspeaker is first fed with a continuous sound signal at a frequency of 1000Hz. The sound pressure created by the loudspeaker is measured and this becomes the reference level sound pressure. Then various sound frequencies are fed to the speaker and their sound levels are measured. Sound pressure will increase or decrease at various frequencies as compared to 1000Hz. The differences are specified as ± dB.

A loudspeaker is accepted as efficient if the sound pressure level variation is equal to or below 4 for the stated frequency range. Thus a speaker specified as 20-20,000Hz ±2.5dB is more efficient than a speaker specified as 20-20,000Hz ± 3.5dB.

Attached Images

SoundPressureLevel_Formula.png (1.7 KB, 107 views)

See the data below:
Frequency 28-24k @ -6dB
Frequency Response (–3dB) 32Hz – 40kHz.It doesnt have upper limit. Then how will we interpret it? Is there any influence for sensitivity on efficiency?

A speakers specification will be, amongst other things, given as follows.

Frequency response 38Hz - 22kHz ±3dB on reference axis
Frequency range -6dB at 30Hz and 50kHz
Sensitivity 90dB spl (2.83V, 1m)

I have already explained frequency response and sensitivity. The frequency response is 'overall;. In other words that represents the frequencies that the loudspeaker can handle.

Frequency range measures the frequency range within which the loudspeaker will not distort the sound beyond a certain limit. This is measured in amplitude and also represented by decibel or dB.

When you increase or decrease the volume, you do not want the loudspeaker to change the 'mix' of tones thereby destroying the timbre of voices and instruments. A music consists of number of voices and instruments each playing at a different frequency. If you record a music, with all these frequencies at the same volume, an ideal loudspeaker should reproduce the music without any change in the volume of individual frequencies across the full range of frequencies that the loudspeaker is capable of reproducing. This is called a loudspeaker having a flat response. Such loudspeaker do not exist. So sound engineers have settled for a amplitude figure that they think is tolerable and this is -9dB. In other words, any frequency reproduced more than 9dB down from the its original frequency will matter little to the sound. The speaker specs above say that this speaker loses 6 dB (or -6dB) on any one or more frequency at 30Hz and at 50Hz. What this means is that at 30Hz, some of the high frequency sounds may be reproduced with 6 dB less amplitude and vice versa. That is why this amplitude drop is shown as a negative figure. (-6dB)

As long as the loudspeakers frequency range covers the frequency response, and the amplitude drop is between 0 to -9dB, the loudspeaker will sound good in terms of imaging and delivering all the frequencies of the recording.

I have chosen these specific terms as different speaker manufacturers show these specification in different ways and the reader gets confused. The specs as shown above are used by reliable manufacturers such as B&W, Polk Audio, etc.

Thank you venkat for detailed explanation.
Thank u very much.
Ajithlal

Small correction. The anove statement should read

In other words, any frequency reproduced not more than 9dB down from the its original frequency will matter little to the sound.

Sorry for the slip up. English is fun and, at the same, difficult. If you miss a word, the meaning of the sentence is completely different Sigh !!!!!

But one practical advise..do not go purely by the specs.
-3 dB at 35 hz and at 40Hz for another speaker may not mean much, as more than 50% of what you hear is due to the placement of the speaker and due to room influence (which can add between 3 - 6 db easily to your response)

also 6 db at 28 means nothing in comparison as based on the design of the speaker, the 3 dB point may be 30 Hz or even 40 Hz !!


Funnily these specs are not even taken under standardized conditions as there is no standardisation .
some manufacturers do it in a Anechoic chamber while others do it in different rooms. some folks keep it on stands while others may hang it at a specific height from the ceiling etc etc....


in the end the room you are going to place the speaker in and the synergy it has with the amplifier driving it is what its sound is going to be defined by...not the specs.

so its better to leave the specs as an indicator of range of the speaker and nothing more .

In general what you say is correct. But nowadays, more and more speaker manufacturers are using sophisticated manufacturing techniques, making their own drivers, and designing high end corssovers. Most of the measurements published by respectable manufacturers are not done by themselves but by independent testing houses to which the loudspeakers are sent. Such testing houses with Anechoic chamber are present in US, in Canada, and in Europe. The Canadian testing site is owned by a research wing of the Canadian government and cannot be pressurised to give false information.

Thus the specifications published by manufacturers such as B&W, Wharfedale, Polk, etc., will not be false. Usually the speakers manufactured by such companies will deliver good results if you understand what the specs mean.

This thread was to understand what these specifications mean and should help in shortlisting speakers that you want to audition.

Ultimately, as I have been saying repeatedly, only a personal audition is the final decision maker. But that does not mean you can ignore specifications.

You are correct, paradigm, Energy and PSB (I think Mirage also does it) are 3 manufacturers who do get speakers tested there in an anechoic chamber (ie National Research Council or NRC) . they i guess publish their specs to 2 db wlimits. although testing results of an Anechoic chamber in what it means to the final consumer can be extensively debated as it is of use (Actually great use) during its design. and these are truly very well measuring speakers


On the other hand Proac, does not even give db limits for their specs..aparently it is a 10db limits !. but again they are fantastic speakers. the reason given is that room parameters are so varying that specs mean nothing much (Which is what incidentally even Bose says !)

Finally polk does publish specs with 3dB limits and Mathew Polk is a true scientist in audio there is no disputing his results

but all three cases above have specs measured unders different considitions and give theior specs differently !

to reiterate i am not disputing the correctness of the Specs..only that the content of what they give does not as mean much

In case manufacturers do give us a response curve and a Impedence/phase curve and possibly even the dispersion data, we would be able to read far more into the spec (Although personal audition is still the most important)
eg the stereophile measurements are really nice
Stereophile: B&W Nautilus 805 loudspeaker

soundstage network also does do a good measurement, although the methodology they follow is again different

any frequency response above 20Khz or so actually goes waste, and i'd rather think it is useless to buy speakers with frequency response above that because human ear can only listen upto 20Khz, CDs have recordings upto 20.5KHZ only. the lower end of frequency reponse is more important, generally a frequency response on lower end of 40Hz will do the job for most of the audio listeners (2ch & 5.1/7.1), but in order to get those deep (i call it Ear Filling, Clothes Shivering) effects you'll need speakers that go even lower to say 30 Hz or so (in case you buy a subwoofer, it'll go even below that to say 24hz).

Sensitvity is a more simple issue, higher the sentivity of speaker less is the power going to be required by it to achive certain levels of loudness as compared to a speaker with compartively low sensitivity.
generall speakers with sensitivity above 90db are considered good, pro grade speaker have sensitivty above 100db.
Klipsch are extremely sensitive and powerfull speakers in my opinion.

ps: frequncy and db's are not interrelated